The present invention relates to a technology for detecting a micro-relief such as a minute projection and a minute concavity, which exist on recording surfaces of a magnetic disk, and more particularly to a technology for detecting a micro-relief on the basis of a deviation value of a frequency or phase of a read signal.
A magnetic head of a magnetic disk drive flies over a magnetic disk so as to write or read data with an extremely slight gap being kept between the magnetic head and a magnetic layer of the magnetic disk. The gap between the magnetic head and the magnetic layer is called “flying height”. In order to increase the recording density, it is necessary to make the flying height as low as possible, and also to keep the flying height constant. For this purpose, it is desirable that surfaces of the magnetic disk be flat. However, because time and cost constraints are placed on a manufacturing process, it is difficult to make the surfaces of the magnetic disk completely flat as a surface finish. In actuality, a minute projection and a minute concavity inevitably remain.
In recent years, the flying height is reduced to about 10 nm, and accordingly an influence of a micro-relief exerted on write or read operation becomes larger. If a read head is a GMR head or a MR head, which uses a GMR (giant magnetoresistive effect) element or an MR (magnetoresistive effect) element respectively, data cannot be read when a collision of the read head with a minute projection causes the temperature of the GMR element or the MR element to increase, which is, what is called, a phenomenon of thermal asperity (hereinafter referred to as TA). On the minute concavity, the flying height substantially increases, and accordingly a degree of magnetic coupling between the magnetic head and the magnetic layer becomes weak. As a result, sufficient magnetization cannot be achieved.
In addition, a protective layer for preventing corrosion is formed on a surface of a magnetic head element. If the magnetic head collides with a minute projection on a surface of the magnetic disk, which causes the protective layer to be worn away, the magnetic head element becomes more subject to corrosion. Moreover, if a lubricating protective layer on the surface of the magnetic disk comes off as a result of the collision of the magnetic head with the minute projection, there is a case where the height of the minute projection increases, or a case where a fragment of the lubricating protective layer which has come off adheres to another part of the magnetic disk, and consequently a new minute projection is formed.
Japanese Patent Application Laid-Open No. Hei 9-251727 (Patent document 1) describes that when a MR head collides with a surface of a magnetic disk, a signal waveform whose signal amplitude (output level) suddenly fluctuates occurs in a read signal. Japanese Patent Application Laid-Open No. Hei 11-66709 (Patent document 2) discloses a technology in which before shipment of a magnetic disk drive, a MR head is caused to collide with a minute projection so as to detect the occurrence of TA, and then defect registration of sectors relating to a position of the projection is performed. Japanese Patent Application Laid-Open No. Hei 9-45009 (Patent document 3) discloses a PLL circuit for, even if an abnormal read signal caused by a defect of a disk is inputted, preventing an oscillation frequency of a synchronization pulse signal from greatly deviating from a specified value. The PLL circuit includes: a phase-locked loop circuit for detecting a phase error between a read signal and a synchronization pulse signal during data read operation, and for executing phase synchronization operation of the synchronization pulse signal so that this phase error is corrected; and a frequency pulling loop circuit for executing frequency pulling operation so that a frequency error with the synchronization pulse signal is corrected.
Japanese Patent Application Laid-Open No. Hei 10-233070 (Patent document 4) discloses a technology for directly detecting, by use of a gap detecting circuit, fluctuations in gap between a magnetic disk and a magnetic head as fluctuations in DC potential by amplifying a signal from the magnetic head using an amplifier, which is directly connected, without passing the signal through a coupling capacitor. In addition, according to the patent document 4, it is possible to keep the magnetic spacing constant by controlling the height of the magnetic head, more specifically, by controlling a gap between the magnetic disk and the magnetic head using a gap control circuit, so that the DC potential equivalent to the detected gap is kept at a constant value.
Heretofore, the MR head is actually caused to contact a minute projection so that TA occurs, and thereby the minute projection on a surface of a magnetic disk is detected before shipment. By use of this method, it is possible to detect a minute projection whose height is high enough to contact the MR head. However, after the use of a magnetic disk drive is started, there is a possibility that a minute projection whose height is not high enough to cause TA before shipment will become a kernel, and then particles generated as a result of the head/slider's contacting a surface of the magnetic disk, particles remaining inside, and the like, will gradually adhere to the kernel so that the kernel grows up to the size which will cause TA.
If the MR head collides with a minute projection after user data is written to a data sector, and consequently TA occurs, user data cannot be read out from the data sector. In addition, there is a possibility that a fragment which has come off from the surface of the magnetic disk as a result of the collision will adhere to another part of the magnetic disk, and will then become a kernel of a new minute projection. Moreover, there is also a possibility that the magnetic head will be damaged. Therefore, in order to avoid a collision, it is desirable that even if a minute projection before shipment is not so large that causes TA, such a minute projection be detected and eliminated. Additionally, after the shipment, it is desirable to avoid a collision of the magnetic head with a minute projection as much as possible.
Furthermore, in the past, even if a data sector contains a minute concavity, if the data sector passes a write/read test before shipment, the data sector was used just as it is. Since the flying height substantially increases on a minute concavity, in order to perform stable write and read operation, it is desirable disallow the use of the data sector beforehand if the depth thereof is greater than or equal to a specified value.
However, in the case of the conventional magnetic disk drives, it was difficult to detect a minute concavity, and a minute projection whose height is not high enough to cause TA. In addition, there was no effective method for, after the shipment of the magnetic disk drive, and after user data has been written, avoiding a situation in which a minute projection grows up, which eventually causes TA.
Embodiments in accordance with the present invention have been made to solve the above problems, and an object of embodiments of the present invention is to provide a magnetic disk drive that is capable of detecting a micro-relief formed on a surface of a magnetic disk. Another object of embodiments in accordance with the present invention is to provide a magnetic disk drive that is capable of detecting a micro-relief to perform stable read and write operation. Still another object of embodiments in accordance with the present invention is to provide a method for registering a defective sector, and for controlling the flying height, by measuring a deviation value of a frequency or phase to detect a micro-relief in a magnetic disk drive.